Didnt help much with Prince John, now did it?
A king never rules long from afar.
There is a reason for that. It wouldnt matter if Jesus H. Christ himself came down from the heavens, turned every square inch of land on earth into a lush, fertile paradise, cured every disease, and ensured that every man woman and child had not only enough to eat, but enough to feel totally satisfied in all things.
He would be hated by many, and many would seek to kill him. There are people who make their livelihoods out of human suffering, and such a clearly benevolent and clearly magical benefactor in power would be completely intolerable to them. Such people judge their own worth by the suffering of others, consciously or unconsciously.
(The wealthy man feels content, that his wealth is a self-evident manifestation of his superiority. He lives better, because he is wealthy. He is wealthy, because he is simply better than those less endowed with riches. If suddenly thrust into a world where his riches could not buy him anything, and he no longer lived better than everyone else, he would suffer an existential crisis. Also, The professional warrior/soldier. Without an enemy to fight, he is nothing. A world without conflict is a world where he has no purpose, and no meaning. He would choose to create wars, simply to fight in them, rather than face this. There are many others. The doctor, without sick and injured. The priest, when God is standing RIGHT THERE, and takes all audiences. The philosopher, when the creator of the universe tells all. Many hands would vie for his death.)
The only effective deterrent is to invoke irrational fear in these would-be assassins. To be larger than life, to the point where people are afraid to even whisper, or be seen by others to harbor even a trace of disloyalty. You basically need to get 1984 on their asses.